This invention relates to glass manufacturing and more particularly to an improved method and apparatus for melting glass. In particular, this invention relates to an improved method and apparatus for heating molten glass and the like through the Joule effect which includes zone temperature control and pulsing of an electrical energy through the temperature control zone.
Typically, glass is generally made by melting a batch of raw glass materials in a furnace. The furnace may be heated by a fossil fuel burner, by electricity, or by a combination of burner and electricity as described in the U.S. Pat. No. 3,885,945 to Rees, deceased et al. Electrically heated glass furnaces include a melting chamber or tank for holding a batch-molten glass body. Two or more electrodes are submerged in the molten glass for heating the glass by the Joule effect when electric voltage is applied between the electrodes. Raw glass batch supplied to the tank floats on the surface of the molten glass and is melted. The batch acts as an insulator both for heat and electricity. Heat losses at the bottom and side walls of the furnace tend to produce a temperature profile through the glass in the vertical plane which peaks nearer the upper surface of the melt than the bottom. Molten glass has a negative temperature coefficient of resistance, hence the molten glass tends to have a lower resistance in upper regions of the melt. Electric current magnitudes are greater in the low resistance molten glass in the upper portion of the melt, thus causing the greatest heat to be developed in these portions. The molten glass is removed from the tank in a region remote from the batch, usually at a submerged throat located in the bottom of the tank.
It is desirable to maintain the temperature of the molten glass at predetermined levels as the molten glass flows through the furnace. If glass on one side of the furnace becomes too hot, the lower viscosity of the glass causes channelization and non uniform glass may be discharged from the furnace. If a "hot spot" in the furnace becomes too intense, the glass may blister and seed. Also, the "hot spot" affects the temperature profile in the flow direction of the molten glass.
It is also desirable that the molten glass be somewhat cooler as it flows from the furnace. Maintenance of a specific output temperature is necessary in order to obtain the desired viscosity in the glass flowing from the furnace. The output temperature, and also the viscosity of the molten glass, is effected by a "hot spot" at an upstream located in the furnace.
The temperature stability problem in electrically heating molten glass was recognized in U.S. Pat. No. 3,836,689 to Holler et al. In this patent a plurality of control thermal zones were established transverse to the direction of glass flow through the furnace. For each thermal zone electrical circuits supplied power to submerged electrodes heating the glass. A control circuit was provided to maintain constant the square of the average current in the circuit. If a predetermined permissible temperature variation was exceeded in the molten glass in any thermal zone, power to all electrodes for such thermal zone was either reduced or interrupted. The thermal zones could also be interdependent in order to maintain a desired temperature profile in the glass flow direction. Such furnace requires great amounts of both fossil fuel and electrical energy.
Accordingly, it is a preferred object of the present invention to improve the method and apparatus for continuously melting glass. Another object is to provide an improved method for increasing heat recovery and heat utilization efficiency in glass melting furnaces.
Another object is to provide an improved method for electrically heating molten glass by the Joule effect.
Other objects and advantages of the invention will become apparent from the following detailed description with reference being made to the accompanying drawings.